Inkjet recording is a recording system for forming characters or images, comprising directly jetting ink droplets from a very fine nozzle to a recording medium, and depositing the ink droplets on the recording medium. This system has some advantages such that not only the usable device shows excellent operability at a low level of noise, but also the coloration is facilitated and a plain sheet of paper can be used as a recording medium. Therefore, in recent years, this system has been widely used. Also, in recent years, pigment inks have been used as an ink for inkjet printers in order to improve water resistance and light fastness.
Since water-based inks in which a pigment is used as a colorant contain an aqueous dispersion containing the pigment and a water-insoluble polymer, it is important to improve storage stability.
As a means of improving storage stability, it is considered to be effective to increase adsorption capacity of a water-insoluble polymer for a pigment, and to incorporate the pigment sufficiently into the water-insoluble polymer. For this purpose, a macromer, a monomer having a long-chained alkyl group or the like has been used as a constitutive component of the polymer (see, e.g., Japanese Patent Laid-Open No. 2002-338783).
The macromer includes a styrene homopolymerized macromer and a copolymerized macromer of styrene and other monomer, having a polymerizable functional group at one end, a silicone macromer, and the like (see, e.g., Japanese Patent Laid-Open No. 2002-338783, WO 00/39226 and Japanese Patent Laid-Open No. 2001-123097).
However, with an increase in the copolymerizing content of especially the macromer of the water-insoluble polymer, there are some disadvantages such as an increase in viscosity of the aqueous dispersion containing a pigment, and storage stability of the water-based ink.